If a participle merely describes a state, then it should be parsed as an adjective if alone, or you may view a whole participial phrase as adverbial if it describes manner or location.
She is wearing a pink sleepsuit with the word 'Itsy' printed on the front, and she is propped up against a darker pink cushion on the sofabed in the room we call the study. — Charles Fernyhough, The Baby In The Mirror, 2010.
The participial phrase describes where and how this baby is sitting. Perhaps one of her parents propped her up in that position or she somehow managed to do it herself, but there is no agent, implied or explicit, and how she got into this position is not the least bit topical.
If the participle describes an action:
Provide a rationale for why an infant's bottle should not be propped up during a feeding. — Lynn R Marotz, Health, Safety, and Nutrition for the Young Child, 2011.
then the participle is a constituent part of a verb phrase in the passive voice. The embedded question here is asking why a particular action — propping up a baby bottle during feeding — should not be performed. The agent is implied: “you” or any person bottle feeding an infant.
Your sentence is identical to the first example: the slippers were propped up just as the baby girl on the pink cushion. It describes a state, not a completed action.
There is far less ambiguity when the agent is explicit:
He was surprised. adjective
He was surprised by the man at the front door. passive